U.S. patent number 7,414,001 [Application Number 11/705,314] was granted by the patent office on 2008-08-19 for sag control of isopipes used in making sheet glass by the fusion process.
This patent grant is currently assigned to Corning Incorporated. Invention is credited to John D. Helfinstine, Daniel J. Liebner, John L. Martin, Dean V. Neubauer, William R. Powell.
United States Patent |
7,414,001 |
Helfinstine , et
al. |
August 19, 2008 |
Sag control of isopipes used in making sheet glass by the fusion
process
Abstract
Isopipes for use in making sheet glass by a fusion process are
provided which exhibit reduced sag. The isopipes are composed of a
zircon refractory which has a mean creep rate (MCR) at 1180.degree.
C. and 250 psi and a 95 percent confidence band (CB) for said mean
creep rate such that the CB to MCR ratio is less than 0.5, the MCR
and the CB both being determined using a power law model. The
zircon refractory can contain titania (TiO.sub.2) at a
concentration greater than 0.2 wt. % and less than 0.4 wt. %. A
concentration of titania in this range causes the zircon refractory
to exhibit a lower mean creep rate than zircon refractories
previously used to make isopipes. In addition, the variation in
mean creep rate is also reduced which reduces the chances that the
zircon refractory of a particular isopipe will have an abnormally
high creep rate and thus exhibit unacceptable sag prematurely.
Inventors: |
Helfinstine; John D. (Big
Flats, NY), Liebner; Daniel J. (Corning, NY), Martin;
John L. (Weston, WV), Neubauer; Dean V. (Horseheads,
NY), Powell; William R. (Horseheads, NY) |
Assignee: |
Corning Incorporated (Corning,
NY)
|
Family
ID: |
22949717 |
Appl.
No.: |
11/705,314 |
Filed: |
February 12, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070142207 A1 |
Jun 21, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11294315 |
Dec 5, 2005 |
7259119 |
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10449701 |
May 29, 2003 |
6974786 |
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PCT/US01/45300 |
Nov 30, 2001 |
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60250921 |
Dec 1, 2000 |
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Current U.S.
Class: |
501/106;
65/374.13; 65/195 |
Current CPC
Class: |
C03B
17/064 (20130101); C04B 35/481 (20130101); C04B
2235/3244 (20130101); C04B 2235/96 (20130101); C04B
2235/3232 (20130101); C04B 2235/3409 (20130101) |
Current International
Class: |
C04B
35/48 (20060101); C03B 17/06 (20060101); C04B
35/49 (20060101) |
Field of
Search: |
;501/106
;65/374.13,195 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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50-027850 |
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Sep 1975 |
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JP |
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11-246230 |
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Sep 1999 |
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JP |
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Other References
US 7,041,614, 05/2006, Ames et al. (withdrawn) cited by other .
Kingery et al., "Plastic Deformation, Viscous Flow, and Creep",
Introduction to Ceramics, 2.sup.nd Edition, John Wiley & Sons,
New York, 1976, pp. 704-767. cited by other .
"Flat Glass", Fundamentals of Inorganic Glasses, Academic Press,
Inc., Boston, 1994, Chapter 20, Section 4.2, pp. 534-540. cited by
other .
Draper et al., Applied Regression Analysis, "Two Predictor
Variables", Chapter 4, John Wiley & Sons, New York, 1981, pp.
193-212. cited by other .
English Translation of Japanese Pat. No. 50-027850 (1975). cited by
other.
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Primary Examiner: Group; Karl E
Attorney, Agent or Firm: Klee; Maurice
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No.
11/294,315, filed on Dec. 5, 2005, now U.S. Pat. No. 7,259,119 the
contents of which are incorporated herein by reference, which is a
continuation of U.S. application Ser. No. 10/449,701, filed on May
29, 2003, now U.S. Pat. No. 6,974,786 the contents of which are
incorporated herein by reference, which is a continuation of
International Application No. PCT/US01/45300, filed on Nov. 30,
2001, which was published in English under PCT Article 21(2) on
Jun. 6, 2002 as International Publication No. WO 02/44102. This
application claims the benefit under 35 USC .sctn.119(e) of U.S.
Provisional Application No. 60/250,921, filed on Dec. 1, 2000.
This application is the result of a joint research agreement
between Corning Incorporated and Corhart Refractories Corporation
(now a part of Saint-Gobain Plastics and Ceramics, Inc.).
Claims
What is claimed is:
1. A method of making an isopipe which has a configuration adapted
for use in a fusion process comprising: (a) providing a block of a
zircon refractory which comprises TiO.sub.2 at a concentration
greater than 0.2 wt. %; and (b) forming the isopipe from said
block.
2. The method of claim 1 wherein forming the isopipe comprises
machining the block.
3. The method of claim 1 wherein the block has a length greater
than 1.5 meters.
4. The method of claim 1 wherein the zircon refractory comprises
TiO.sub.2 at a concentration less than 0.4 wt. %.
5. The method of claim 1 wherein the zircon refractory comprises
TiO.sub.2 at a concentration greater than 0.25 wt. % and less than
0.35 wt. %.
6. The method of claim 1 wherein the zircon refractory comprises
TiO.sub.2 at a concentration of about 0.3 wt. %.
7. The method of claim 1 wherein the zircon refractory has a mean
creep rate at 1800.degree. C. and 250 psi of less than
0.7.times.10.sup.-6 inches/inches/hour, where the mean creep rate
is determined using a power law model.
8. The method of claim 1 wherein the zircon refractory has a mean
creep rate (MCR) at 11 800C. and 250 psi and a 95 percent
confidence band (CB) for said mean creep rate such that the CB to
MCR ratio is less than 0.5, said mean creep rate and said 95
percent confidence band being determined using a power law model.
Description
FIELD OF THE INVENTION
This invention relates to isopipes used in the production of sheet
glass by the fusion process and, in particular, to techniques for
controlling the sag which such isopipes exhibit during use.
BACKGROUND OF THE INVENTION
A. The Fusion Process
The fusion process is one of the basic techniques used in the glass
making art to produce sheet glass. See, for example, Varshneya,
Arun K., "Flat Glass," Fundamentals of Inorganic Glasses, Academic
Press, Inc., Boston, 1994, Chapter 20, Section 4.2., 534-540.
Compared to other processes known in the art, e.g., the float and
slot draw processes, the fusion process produces glass sheets whose
surfaces have superior flatness and smoothness. As a result, the
fusion process has become of particular importance in the
production of the glass substrates used in the manufacture of
liquid crystal displays (LCDs).
The fusion process, specifically, the overflow downdraw fusion
process, is the subject of commonly assigned U.S. Pat. Nos.
3,338,696 and 3,682,609, to Stuart M. Dockerty, the contents of
which are incorporated herein by reference. A schematic drawing of
the process of these patents is shown In FIG. 1. As illustrated
therein, the system includes a supply pipe 9 which provides molten
glass to a collection trough 11 formed in a refractory body 13
known as an "isopipe."
Once steady state operation has been achieved, molten glass passes
from the supply pipe to the trough and then overflows the top of
the trough on both sides, thus forming two sheets of glass that
flow downward and then inward along the outer surfaces of the
isopipe. The two sheets meet at the bottom or root 15 of the
isopipe, where they fuse together into a single sheet. The single
sheet is then fed to drawing equipment (represented schematically
by arrows 17), which controls the thickness of the sheet by the
rate at which the sheet is drawn away from the root. The drawing
equipment is located well downstream of the root so that the single
sheet has cooled and become rigid before coming into contact with
the equipment.
As can be seen in FIG. 1, the outer surfaces of the final glass
sheet do not contact any part of the outside surface of the isopipe
during any part of the process. Rather, these surfaces only see the
ambient atmosphere. The inner surfaces of the two half sheets which
form the final sheet do contact the isopipe, but those inner
surfaces fuse together at the root of the isopipe and are thus
buried in the body of the final sheet. In this way, the superior
properties of the outer surfaces of the final sheet are
achieved.
As is evident from the foregoing, isopipe 13 is critical to the
success of the fusion process. In particular, the dimensional
stability of the isopipe is of great importance since changes in
isopipe geometry affect the overall success of the process. See,
for example, Overman, U.S. Pat. No. 3,437,470, and Japanese Patent
Publication No. 11-246230.
Significantly, the conditions under which the isopipe is used make
it susceptible to dimensional changes. Thus, the isopipe must
operate at elevated temperatures on the order of 1000.degree. C.
and above. Moreover, in the case of the overflow downdraw fusion
process, the isopipe must operate at these elevated temperatures
while supporting its own weight as well as the weight of the molten
glass overflowing its sides and in trough 11, and at least some
tensional force that is transferred back to the isopipe through the
fused glass as it is being drawn. Depending on the width of the
glass sheets that are to be produced, the isopipe can have an
unsupported length of 1.5 meters or more.
To withstand these demanding conditions, isopipes 13 have been
manufactured from isostatically pressed blocks of refractory
material (hence the name "iso-pipe"). In particular, isostatically
pressed zircon refractories have been used to form isopipes for the
fusion process. As known in the art, zircon refractories are
materials composed primarily of ZrO.sub.2 and SiO.sub.2, e.g., in
such materials, ZrO.sub.2 and SiO.sub.2 together comprise at least
95 wt. % of the material, with the theoretical composition of the
material being ZrO.sub.2.SiO.sub.2 or, equivalently, ZrSiO.sub.4.
Even with such high performance materials, in practice, isopipes
exhibit dimensional changes which limit their useful life. In
particular, isopipes exhibit sag such that the middle of the
unsupported length of the pipe drops below its outer supported
ends. The present invention is concerned with controlling such
sag.
A primary contributor to the sag of an isopipe is the creep rate
{dot over (.epsilon.)}=d.epsilon./dt of the material from which it
is made. As known in the art, for many materials, creep rate as a
function of applied stress .sigma. can be modeled by a power law
expression of the following form: {dot over
(.epsilon.)}=A.sigma..sup.nexp(Q/T) (1) where T is temperature and
A, n, and Q are material dependent constants. See Kingery et al.,
"Plastic Deformation, Viscous Flow, and Creep," Introduction to
Ceramics, 2nd edition, John Wiley & Sons, New York, 1976,
704-767 and, in particular, equation 14.9. Being the time
derivative of strain, the units of creep rate are
length/length/time. Because in equation (1) creep rate varies as
stress raised to a power, i.e., .sigma..sup.n, the use of equation
(1) will be referred to herein as the "power law model."
Lowering the creep rate of the material used to make an isopipe
results in less sag during use. As discussed in detail below, in
accordance with certain aspects of the invention it has been found
that the sag of an isopipe can be reduced by forming the isopipe
from an isostatically pressed zircon refractory having a titania
(TiO.sub.2) content which is greater than 0.2 wt. % and less than
0.4 wt. %, e.g., a TiO.sub.2 content of approximately 0.3 wt. %. In
particular, it has been found that such a zircon refractory
exhibits a lower mean creep rate than zircon refractories used in
the past to from isopipes and having a titania content of about 0.1
wt. %.
In addition, it has also been found that controlling the titania
content of a zircon refractory to be within the above range
significantly enhances the usefulness of the power law model of
equation (1) in modeling the sag of isopipes during use. This
enhanced usefulness results from improved 95% confidence intervals
for the mean creep rates predicted by the model when equation (1)
is evaluated for a particular set of .sigma., T values. Such
improved 95% confidence intervals, in turn, mean that the sag which
an isopipe will exhibit during use can be more accurately modeled
using, for example, a finite element or other modeling technique.
More accurate modeling greatly enhances the ability to develop
improved isopipe designs since numerous designs can be evaluated
theoretically with only the best candidates being selected for
actual construction and testing.
B. Zircon Refractories
As indicated above, the present invention relates to isopipes
composed of a zircon refractory having a titania concentration
within specified limits. Corhart Refractories Corporation
(Louisville, Ky.) offers a number of zircon refractories containing
varying amounts of TiO.sub.2. For example, Corhart's ZS-835 product
is specified to contain 0.2 wt. % TiO.sub.2, its ZS-835HD product
0.4 wt. %, its Zircon 20 product 0.7 wt. %, and its ZS-1300 product
1.2 wt. %.
As a raw material, zircon can have varying amounts of titania. For
example, U.S. Pat. No. 2,752,259 reports that the zircon used in
its examples had 0.34 wt. % TiO.sub.2, while the zircon used in
U.S. Pat. No. 3,285,757 had 0.29 wt. % TiO.sub.2. U.S. Pat. Nos.
3,347,687 and 3,359,124 each describe zircons having TiO.sub.2
concentrations of 0.2 wt. %. In addition to being naturally present
in zircon as a raw material, TiO.sub.2 can also be a component of
clays used in producing zircon refractories. See U.S. Pat. Nos.
2,746,874 and 3,359,124.
Other discussions of the use of titania in zircon products can be
found in Goerenz et al., U.S. Pat. No. 5,407,873 which discloses
(1) the use of phosphorus compounds to improve the corrosion
resistance of zirconium silicate bricks and (2) the use of titanium
dioxide as a sintering aid in the manufacture of such bricks.
Although the patent states that sintering can be improved by adding
between 0.1 wt. % and 5 wt. % of titanium dioxide, all of the
examples of the patent use more than 1 wt. % of titanium dioxide
and the patent's preferred composition consists of 98 wt. %
zirconium silicate, 1.5 wt. % titanium dioxide, and 0.5 wt. % of a
phosphorous compound.
Wehrenberg et al., U.S. Pat. No. 5,124,287 relates to the use of
zirconia in particle form to improve the thermal shock resistance
of zircon refractories. Titania is employed to enhance grain growth
during sintering. The patent claims titania concentrations between
0.1 wt. % and 4 wt. %. The preferred titania concentration is 1 wt.
%, and when blistering is a problem, only 0.1 wt. % titania is
used. The patent states that "grog" having a titania concentration
of 0.2 wt. % was used as a starting material for some of its
examples.
Significantly, none of the foregoing disclosures regarding the use
of titania in zircons relates to employing titania concentration as
a means to control the creep rate of a zircon refractory, or to
enhance the ability of a power law model to represent the material,
or to achieve the ultimate goal of reducing the sag of an isopipe
made of a zircon refractory.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of this invention to
provide improved isopipes for use in the fusion process. More
particularly, it is an object of the invention to provide isopipes
that exhibit less sag than existing isopipes.
To achieve the foregoing and other objects, the invention in
accordance with a first aspect provides isopipes which comprise a
zircon refractory that exhibits a lower creep rate than the zircon
refractories previously used to produce isopipes.
In accordance with a second aspect, the invention provides isopipes
which comprise a zircon refractory that in comparison to zircon
refractories previously used to produce isopipes, has a creep rate
that can be modeled more accurately by a power law model.
In accordance with a third aspect, the invention provides isopipes
comprising a body having a configuration adapted for use in a
fusion process, said body comprising a zircon refractory which
purposely comprises TiO.sub.2 at a concentration greater than 0.2
wt. % and less than 0.4 wt. %, preferably greater than 0.25 wt. %
and less than 0.35 wt. %, and most preferably about 0.3 wt. %.
In accordance with a fourth aspect, the invention provides isopipes
comprising a body having a configuration adapted for use in a
fusion process, said body comprising a zircon refractory which has
a mean creep rate (MCR) at 1180.degree. C. and 250 psi of less than
0.7.times.10.sup.-6 inches/inches/hour, preferably less than
0.6.times.10.sup.-6 inches/inches/hour, and most preferably less
than 0.5.times.10.sup.-6 inches/inches/hour, where the MCR is
determined using a power law model, i.e., a power law model fit to
experimental data.
In accordance with this fourth aspect, the zircon refractory also
preferably has a MCR at 1180.degree. C. and 1000 psi of less than
5.times.10.sup.-6 inches/inches/hour and more preferably less than
3.times.10.sup.-6 inches/inches/hour, where again the MCR is
determined using a power law model.
In accordance with a fifth aspect, the invention provides isopipes
comprising a body having a configuration adapted for use in a
fusion process, said body comprising a zircon refractory which has
a MCR at 1180.degree. C. and 250 psi and a 95 percent confidence
band (CB) for said MCR such that the CB to MCR ratio is less than
0.5, the MCR and the CB both being determined using a power law
model. In accordance with these aspects of the invention, the CB to
MCR ratio at 1180.degree. C. and 1000 psi is also preferably less
than 0.5, where again the MCR and the CB values used to calculate
the CB to MCR ratio at said temperature and stress level are
determined using a power law model.
In accordance with a sixth aspect, the invention provides a method
for reducing the sag of an isopipe used in a fusion process that
produces glass sheets comprising forming said isopipe from a zircon
refractory which purposely comprises TiO.sub.2 at a concentration
greater than 0.2 wt. % and less than 0.4 wt. %, preferably greater
than 0.25 wt. % and less than 0.35 wt. %, and most preferably about
0.3 wt. %.
The above first through sixth aspects of the invention can be used
separately or in all possible combinations. For example, the
compositional limitations of the third and sixth aspects of the
invention (including the base, preferred, and most preferred values
of those limitations) can be combined with the mean creep rate
limitations of the fourth aspect of the invention (including the
base, preferred, and most preferred values of those limitations)
and/or with the CB to MCR ratio limitations of the fifth aspect of
the invention (including the base and preferred pressure values of
those limitations). Similarly, the mean creep rate limitations of
the fourth aspect of the invention (including the base, preferred,
and most preferred values of those limitations) can be combined
with CB to MCR ratio limitations of the fifth aspect of the
invention (including the base and preferred pressure values of
those limitations).
As used in this specification and in the claims, the term "isopipe"
means any sheet forming delivery system used in a fusion process
which produces flat glass wherein at least a part of the delivery
system comes into contact with the glass just prior to fusion,
irrespective of the configuration or the number of components
making up the delivery system. Also, the MCR and CB values are
determined using standard statistical techniques for calculating
such values from the fit of an equation such as the power law model
to measured data. See, for example, Draper et al., Applied
Regression Analysis, John Wiley & Sons, New York, 1981,
193-212.
Further, the word "purposely" when used in connection with
TiO.sub.2 concentrations means that the TiO.sub.2 concentration is
intentionally selected to control isopipe sag and is not merely a
TiO.sub.2 concentration which one or more zircons (including
zircons for making isopipes) may have had as a result of
compositional variations without being the result of a conscious
intention to control isopipe sag and/or to improve the ability of a
power law model to represent the creep rate of zircon used in an
isopipe.
Additional features and advantages of the invention will be set
forth in the detailed description which follows, and in part will
be readily apparent to those skilled in the art from that
description or recognized by practicing the invention as described
herein. It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework for understanding the nature and character of the
invention as it is claimed. The accompanying drawings are included
to provide a further understanding of the invention, and are
incorporated in and constitute a part of this specification. The
drawings illustrate various aspects of the invention, and together
with the description serve to explain the principles and operation
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing illustrating a representative
construction for an isopipe for use in an overflow downdraw fusion
process for making flat glass sheets.
FIGS. 2A and 2B are three dimensional plots showing experimentally
measured creep rate as a function of temperature and stress for
zircon specimens having TiO.sub.2 concentrations of 0.12 wt. % and
0.30 wt. %, respectively.
FIGS. 3A and 3B are plots illustrating the differences in creep
rate variability at a temperature of 1180.degree. C. for 0.12 wt. %
TiO.sub.2 versus 0.30 wt. % TiO.sub.2 for applied stresses of 250
psi and 1000 psi, respectively.
DETAILED DESCRIPTION OF THE INVENTION
As discussed above, the present invention relates to the use of
zircon refractories to produce isopipes for use in a fusion process
where the zircon refractory has a TiO.sub.2 content greater than
0.2 wt. % and less than 0.4 wt. %.
Such a TiO.sub.2 content causes the isopipe to exhibit reduced sag
as a result of the refractory having a lower mean creep rate than
zircon refractories currently used in the art. For example, the
zircon refractory can have a mean creep rate at 1180.degree. C. and
250 psi substantially less than 0.5.times.10.sup.-6
inches/inches/hour.
In addition, such a TiO.sub.2 content also causes the refractory to
have a 95% confidence band (CB) for said mean creep rate (MCR)
which is less than 50% of the mean creep rate, i.e., CB/MCR<0.5.
Such a confidence band reduces the chances that the zircon
refractory of a particular isopipe will have an abnormally high
creep rate and thus cause the isopipe to have a short lifetime as a
result of exhibiting unacceptable sag prematurely.
The TiO.sub.2 content of a zircon refractory can be determined
using various techniques known in the art. For example, the content
can be determined by means of an X-ray fluorescence analysis (XRF).
The titania content of the refractory can be adjusted so that the
final product has the desired TiO.sub.2 content by incorporating
TiO.sub.2 as needed in the batch materials used to prepare the
refractory. Thereafter, the refractory can be prepared in
accordance with techniques currently known in the art or with
improved techniques which may be developed in the future.
Similarly, isopipes can be prepared from the zircon refractories of
the invention using techniques currently known in the art or with
improved techniques which may be developed in the future.
Typically, the isopipe will be prepared by being machined from a
single block of the zircon refractory, although other approaches
can be used if desired.
Without intending to limit it in any manner, the present invention
will be more fully described by the following examples.
Lots of zircon refractories containing 0.12 wt. % or 0.30 wt. %
TiO.sub.2 were obtained from Corhart Refractories Corporation
(Louisville, Ky.). Each lot represented a separate firing and
typically included multiple blocks of material of suitable
dimensions to produce an isopipe, i.e., the blocks had lengths
greater than 1.5 meters.
Creep rate tests were performed on 117 specimens taken from blocks
having 0.12 wt. % TiO.sub.2 and 142 specimens taken from blocks
having 0.30 wt. % TiO.sub.2. A three point flexure technique was
used to determine creep rates in which a bar of the material being
tested was supported at its ends and loaded at its center. The
applied stress in pounds per square inch (psi) was determined in
accordance with conventional procedures as set forth in ASTM C-158.
In particular, applied stress .sigma. was determined from the
relation: .sigma.=3ALSS/(2SWSH.sup.2) where AL=applied load,
SS=support span, SW=specimen width, and SH=specimen height.
The bar was heated and its flexure as a function of time was
measured. A midspan deflection rate was obtained by calculating the
slope of the resulting deflection versus time plot once steady
state conditions for the particular load and temperature had been
reached. In particular, the midspan deflection rate was determined
for the "secondary creep" portion of the strain versus time curve.
See, for example, the Kingery et al. text cited above at pages
707-709.
Creep rates {dot over (.epsilon.)} were then obtained from the
relation: {dot over (.epsilon.)}=DR2SH/SS.sup.2 where SH and SS are
as defined above and DR=midspan deflection rate.
FIG. 2A and FIG. 2B are three dimensional plots of the creep rate
values obtained in this way for the 0.12 wt. % TiO.sub.2 and 0.30
wt. % TiO.sub.2 specimens, respectively. The reduction in the
scatter of the data achieved by the change in TiO.sub.2
concentration is immediately evident from these figures. In terms
of producing isopipes which will have repeatable creep properties,
the data of these figures show that a zircon refractory having a
TiO.sub.2 content of around 0.3 wt. % is clearly much better than
one having a TiO.sub.2 content around 0.1 wt. %.
The power law model of equation (1) was fit to the data of FIG. 2A
and to that of FIG. 2B using a commercial data analysis package,
namely, "TableCurve 3D: Automated Surface Fitting and Equation
Discovery," Version 3.0 for Windows.RTM. 95 & NT, software and
documentation, SPSS Inc., Chicago, 1997 (hereinafter the "TABLE
CURVE 3D program"). The values of the material dependent constants
A, n, and Q obtained in this way for the two cases are set forth in
Table 1.
Using these constants and the TABLE CURVE 3D program, mean creep
rates and 95% confidence bands were determined for a temperature of
1180.degree. C. and a stress of 250 psi, which are representative
of the temperatures and stress levels which an isopipe will
typically experience during use. The results of this analysis are
shown in FIG. 3A.
Two important facts are evident from this figure. First, the mean
creep rate has been substantially reduced as a result of the
increase in TiO.sub.2 content from 0.12 wt. % to 0.30 wt. %. This
means that isopipes composed of zircon refractories having higher
TiO.sub.2 than previously used will exhibit less sag during use, a
highly desirable result. Moreover, the size of the 95% confidence
band has also been substantially reduced by the increase in
TiO.sub.2 content. This means that an individual isopipe made from
an individual block of a zircon refractory is more likely to have
its creep rate closer to the predicted mean creep rate when the
TiO.sub.2 content of the material is increased than when it is not
increased, another highly desirable result since predictability in
a manufacturing setting makes for more efficient planning and
operation.
To further demonstrate the controlling effect which TiO.sub.2
content has on creep rate, mean creep rates and 95% confidence
bands were also determined for a stress of 1000 psi, again using
the constants of Table 1 and the TABLE CURVE 3D program. The
results are shown in FIG. 3B. The reduction in mean creep rate
achieved by increasing the TiO.sub.2 content is even greater at
this higher stress level.
Table 2 summarizes the results of using the TABLE CURVE 3D program
to determine mean creep rates and 95% confidence bands for the data
of FIG. 2. Zircon refractories having a TiO.sub.2 content above and
below the 0.3 wt. % value used to generate this data will exhibit
similar MCR and CB values to those shown in Table 2. In particular,
reduced MCR and CB/MCR values compared to previously used zircon
refractories are achieved when the TiO.sub.2 content of the
refractory is greater than 0.2 wt. %. The improved performance
continues as the TiO.sub.2 content is increased above 0.3 wt. %.
However, oxygen blisters can be generated at the isopipe/glass
interface when the TiO.sub.2 content of the zircon refractory
reaches about 0.4 wt. %. Thus, in accordance with the invention,
the TiO.sub.2 content of the refractory should be above 0.2 wt. %
but below 0.4 wt. %.
Although specific embodiments of the invention have been discussed,
a variety of modifications to those embodiments which do not depart
from the scope and spirit of the invention will be evident to
persons of ordinary skill in the art from the disclosure herein.
The following claims are intended to cover the specific embodiments
set forth herein as well as such modifications, variations, and
equivalents.
TABLE-US-00001 TABLE 1 TiO.sub.2 (wt. %) A n Q 0.12 1.04 .times.
10.sup.12 1.56 -73302 0.30 1.20 .times. 10.sup.14 1.33 -79038
TABLE-US-00002 TABLE 2 MCR TiO.sub.2 T .sigma. (10.sup.-6 CB CB/
Example (wt. %) (.degree. C.) (psi) in/in/hr) (10.sup.-6 in/in/hr)
MCR 1 0.12 1180 250 0.7197 0.5163 to 1.003 0.6763 2 0.30 1180 250
0.4340 0.3500 to 0.5390 0.4355 3 0.12 1180 1000 6.296 4.811 to
8.240 0.5446 4 0.30 1180 1000 2.730 2.210 to 3.380 0.4286 MCR =
mean creep rate CB = 95% confidence band for the MCR
* * * * *